mikey said:
I had an experience many years ago with using a curtain PIR in a
similar type of installation.
Customer wanted protection above his drop ceiling against someone
coming in through the roof. The distance between the hard ceiling and
the drop ceiling was about 3 feet. I mounted a Visonic (forget the
model #) curtain PIR on it's side. I seem to remember it was rated at
35 feet(?). I couldn't get detection at any range further than about
17-18 feet. I wound up having to use more than one unit above the
ceiling, to cover the area that I wanted.
I had to assume that the divergence of the array, between the two
ceilings, got too wide with distance to be able to detect the cross
section of a body perpendicular to the curtain, at anything further
than 17 feet.
When I tested the unit below the ceiling it worked at a greater
distance but not at the maximum distance the unit was rated for. I
found that when the unit was mounted on it's side and you "stood up"
perpendicular to the curtain, the unit wouldn't detect as far away as
it would as if the unit were mounted with the unit mounted properly and
you walked through the curtain in parallel with the array. I assumed
that since the size of the heat source the unit sees is smaller when
mounted on it's side, that the range was somewhat shortened.
That was with Visonics very first curtain unit. I'm sure things have
changed since then. ( don't have much call for curtain units) But it
would be an interesting experiment to see if it still affects the range
when used horizontally.
